A continuous extruder of a plastic product, whether formed of a thermoplastic resin or a thermosetting resin, produces a product that is very hot and soft and has little strength. The product is still quite plastic and must be cooled to achieve structural integrity. The longer the product remains soft, the greater the likelihood of surface damage or even structural damage. Thus, prompt and effective cooling of the extruded product is of substantial importance.
A variety of different cooling arrangements have been employed for continuous plastic extruders. For example, Allan et al. U.S. Pat. No. 2,814,071 discloses a variety of forms of external cooling for a tubular extrusion as it emerges from an extruding die. Thus, this patent refers to external air blast cooling, water spray cooling, or open trough water bath cooling; the patent specifically states that air cooling is preferred. This patent shows an internal air circulation technique used primarily for support of the tube; any cooling effect appears to be only incidental.
Ball U.S. Pat. No. 3,522,377 discloses a rather complex apparatus for internal cooling of an extruded tube with cooling water introduced at a downstream point in the tube by means of a coaxial internal cooling line. A gas stream is used, upstream, to deflect the cooling liquid into a drainage system. No specific external cooling arrangement is disclosed. Carrow U.S. Pat. No. 3,907,961 described another internal cooling arrangement for an extruded tube, using a support cylinder with a spiral external groove to circulate a cooling fluid in contact with the internal surface of the tube. Reifenhauser U.S. Pat. No. 3,990,828 describes another internal cooling apparatus that uses water as the coolant; the cooling apparatus features a plurality of axially spaced baffle discs.
None of these prior art cooling devices provides for effective external cooling of an extruded plastic product by means of a liquid coolant (i.e., water), despite the fact that a liquid coolant is inherently more effective than gas (i.e., air) cooling. The usual air cooling arrangement is inefficient and required an excessively long cooling zone. A water trough cannot cool the upper part of the extrusion, particularly a hollow extrusion. Problems of surface damage and even structural damage remain prevalent, even with the best of systems.